II. Kinematic Complexity in a Rising Velocity Dispersion Proﬁle Around the Cd Galaxy NGC 3311 M

A&A 619, A70 (2018) Astronomy https://doi.org/10.1051/0004-6361/201731737 & c ESO 2018 Astrophysics The Hydra I cluster core II. Kinematic complexity in a rising velocity dispersion profile around the cD galaxy NGC 3311 M. Hilker1, T. Richtler2, C. E. Barbosa1,3, M. Arnaboldi1, L. Coccato1, and C. Mendes de Oliveira3 1 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany e-mail: [email protected] 2 Universidad de Concepción, Concepción, Chile 3 Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Rua do Matão 1226, São Paulo, SP, Brazil Received 8 August 2017 / Accepted 14 August 2018 ABSTRACT Context. NGC 3311, the central galaxy of the Hydra I cluster, shows signatures of recent infall of satellite galaxies from the cluster environment. Previous work has shown that the line-of-sight velocity dispersion of the stars and globular clusters in the extended halo of NGC 3311 rises up to the value of the cluster velocity dispersion. In the context of Jeans models, a massive dark halo with a large core is needed to explain this finding. However, position dependent long-slit measurements show that the kinematics are still not understood. Aims. We aim to find kinematic signatures of sub-structures in the extended halo of NGC 3311. Methods. We performed multi-object spectroscopic observations of the diffuse stellar halo of NGC 3311 using VLT/FORS2 in MXU mode to mimic a coarse “IFU”. The slits of the outermost masks reach out to about 35 kpc of galactocentric distance. We use pPXF to extract the kinematic information of velocities, velocity dispersions and the high-order moments h3 and h4. Results. We find a homogeneous velocity field and velocity dispersion field within a radius of about 10 kpc. Beyond this radius, both the velocities and the velocity dispersion start to depend on azimuth angle and show a significant intrinsic scatter. The inner spheroid of NGC 3311 can be described as a slow rotator. Outside 10 kpc the cumulative angular momentum is rising, however, without show- ing an ordered rotation signal. If the radial dependence alone is considered, the velocity dispersion does not simply rise but fills an increasingly large range of dispersion values with two well defined envelopes. The lower envelope is about constant at 200 km s−1. The upper envelope rises smoothly, joining the velocity dispersion of the outer globular clusters and the cluster galaxies. We interpret this behaviour as the superposition of tracer populations with increasingly shallower radial distributions between the extremes of the inner stellar populations and the cluster galaxies. Simple Jeans models illustrate that a range of mass profiles can account for all observed velocity dispersions, including radial MOND models. Conclusions. The rising velocity dispersion of NGC 3311 apparently is a result of averaging over a range of velocity dispersions related to different tracer populations in the sense of different density profiles and anisotropies. Jeans models using one tracer population with a unique density profile are not able to explain the large range of the observed kinematics. Previous claims about the cored dark halo of NGC 3311 are therefore probably not valid. This may in general apply to central cluster galaxies with rising velocity dispersion profiles, where infall processes are important. Key words. galaxies: clusters: individual: Hydra I – galaxies: elliptical and lenticular, cD – galaxies: haloes – galaxies: individual: NGC 3311 – galaxies: kinematics and dynamics – galaxies: formation 1. Introduction Theclusterenvironmentisclearlyimportantforthebuild-upof such a large halo, either by coalescence of larger galaxies or by the The structure of galaxies can be strongly altered by environ- infall of smaller galaxies or tidal debris of cluster material. This is mental processes, like major and minor mergers, accretion of plausibly also the reason for the immense richness of the globular satellite galaxies, ram pressure stripping and other interactions clustersystems (GCSs)of centralgalaxies (e.g.,Harris et al. 2017) between galaxies and with the intra-cluster medium. A striking and,inthecaseofthisstudy,therichGCSofNGC 3311,thecentral phenomenon in this respect is the existence of very extended galaxy of the Hydra I galaxy cluster (Wehner et al. 2008). stellar haloes around bright central galaxies in galaxy clus- Given that the haloes of massive ellipticals in general grow ters (e.g. Schombert 1987, 1988). Early classification efforts byafactorofabout4inmasssincez = 2(van Dokkum et al.2010), assigned the term “cD” to those galaxies (Matthews et al. 1964; one expects an even higher growth rate in the centres of galaxy Morgan & Lesh 1965) whose haloes can in extreme cases embrace clusters, where the galaxy number density is highest. The more the entire host galaxy cluster as in the case of Abell 1413 (Oemler recent mass growth is dominated by the accretion of low mass sys- 1976; Castagné et al. 2012) frequently also presenting double or tems (minor mergers) which may leave kinematical signatures in multiple nuclei of central galaxies (see Kormendy & Djorgovski the phase space of the outer stellar population of central cluster 1989, for a discussion of the early literature). In the following, we galaxies. These extreme environments, therefore, are suitable to use the term “cD” in this simple sense, irrespective of whether the study the main physical processes that cause the destruction of halo can photometrically identified as a separate entity or not, see infalling galaxies as well as the build-up of the intra cluster light Bender et al.(2015) for a more profound discussion. and the central dark matter halo of a galaxy cluster. Article published by EDP Sciences A70, page 1 of 23 A&A 619, A70 (2018) The extended envelopes make it possible to probe the kine- science data were taken in seven different nights between Jan- matical properties of the galaxy light out to large radii. A rise uary 17th and March 29th, 2012, with FORS2 (in multi-object of the velocity dispersion of the galaxy light is not unusual mode, using the Mask eXchange Unit (MXU)) mounted on UT1 for the most massive early-type galaxies (Newman et al. 2013; at Paranal Observatory. We used the 1400V grism, which cov- Veale et al. 2017). The first measured rising velocity disper- ers the wavelength range 4600–5800 Å. With a slit width of sion profile was reported by Dressler(1979) for the cD galaxy 100 and a dispersion of 0.31 Å pixel−1 we reach a spectral res- IC 1101 in the cluster Abell 2029. In the second known and well olution of R = 2100 at 5200 Å, which translates into a veloc- studied case of NGC 6166, the velocity dispersion stays constant ity resolution of ∼140 km s−1. Six different MXU masks were at a value of 200 km s−1 until a radius of 10 kpc and then steeply −1 necessary to cover the halo around NGC 3311. For each mask rises to high values above 800 km s at large radii, reaching the twilight skyflats were taken, mostly in the same nights as the velocity dispersion of galaxies in the cluster (Carter et al. 1999; science exposures. Those flats are important to calibrate the rel- Kelson et al. 2002; Bender et al. 2015). ative responses between object and sky slits. One of the nearest cD-galaxies is NGC 3311, the cen- The mask design follows an “onion-shell” approach. Each tral galaxy in the Hydra I galaxy cluster. Recent publications mask consists of a half-ring of halo slits, all positioned at about on NGC 3311 are mainly from our group (Ventimiglia et al. the same galactocentric distance from NGC 3311. The typical 2008, 2010, 2011; Coccato et al. 2011; Misgeld et al. 2011; dimensions of the slits are 1 × 500. The combination of 2 × 3 Richtler et al. 2011; Arnaboldi et al. 2012; Barbosa et al. 2016, masks with half-rings of slits at different galactocentric distances 2018). Photometric and kinematical studies using the galaxy on each side of NGC 3311 complete the full onion-shell of halo light, planetary nebulae (PNe) and globular clusters (GCs) slits. This is shown in Fig. B.1. The outermost halo slits reach clearly demonstrate the close connection of the inner galaxy distances of 15000(∼35 kpc) from the galaxy centre. The onion- with its host cluster. Asymmetric light distribution and tidal shell approach allows us to study the stellar population proper- tails are evidences of recent infall processes (Arnaboldi et al. ties at several radii as well as at different azimuthal angles in the 2012). Planetary nebulae show a non-Gaussian velocity distri- halo of NGC 3311. It kind of mimics a coarse IFU. The advan- bution with peaks that are offset from the systemic velocity tage of such an approach is that an “effective IFU field-of-view” (Ventimiglia et al. 2011). The central velocity dispersion of the of about 4 × 4 arcmin is homogeneously covered, which is 16× galaxy light is only 150 km s−1 (but fits to its low surface bright- −1 larger than the so far largest single IFU Multi-Unit Spectroscopic ness) and rises up to almost 400 km s within 10 kpc. The more Explorer (MUSE) at the VLT. distant globular clusters reach an even higher velocity disper- The halo slits avoid point sources and galaxies, except the sion, equal to that of the cluster galaxies (Misgeld et al. 2011; lenticular galaxy HCC 007 south of NGC 3311. The position Richtler et al. 2011). To explain this rise with a simple Jeans angle of the masks and thus slits was chosen such that we have model, one needs a large core of dark matter. The population free sky regions close to the borders of the 7.80 field-of-view composition of NGC 3311 indicates a distinction between the in spatial direction of the slits.

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